Need Help Determing Posi Gear Ratio

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notter

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Hi, I am new to this forum and hoping to get some advice. I would like to put a posi rear end in my 1969 Firebird Convertible. I need help determining the gear ratio. I want this car to be an everyday driver and be able to go on long highway trips. Here is what I know about the car:

1969 Firebird Convertible
400 Engine (Out of a 1977 Pontiac Grand Prix)
ST10 Borg Warner 4-Speed w/following gear ratio:
2.64. 1.75, 1.33, 1

I have to purchase the posi and am asking what ratio I should put in or buy with. Any suggestions would be much appreciated. If I haven't provided enough info, just ask. Thank you. Rick
 
Do you know your current ratio or are you asking how to figure it out? A 3:55 is a nice all around choice.
 
Posi

Thanks for the reply. We don't know what our current is as the rear end is stock and we want to replace it with a posi. The ST10 tranny came out of a 1976 Corvette that had a 3:36 rear axle ratio. Thought that might work but wasn't positive so thought I'd ask.
 
1. Jack up the back of the car
2. Place transmission in neutral, parking brake off.
3. put a chalk mark or piece of tape on driveshaft and either rear tire.
4. turn tire one complete revolution while watching how many times the drive shaft turns. If the driveshaft turns about 3 1/2 times the ratio is about 3:50.

A corvette uses a complete different type of posi it will not fit in your car.
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Posi

Sorry wasn't meaning that I was putting the corvette rear end in our car only saying that the ratio that was in the corvette was a 3:36 and is 3:36 a good ratio? Is it necessary to know what I currently have since it isn't what I am going to have as it will be replaced with a posi?

We are rebuilding this car. The 4-speed has never been in it. We are changing it over from an auto tranny. Currently, the rear axle, motor, tranny, etc., aren't even attached to the car. It is a bucket that we are getting ready to put back together now that the body work is complete (well almost complete).

Sorry, not trying to be difficult. New to this whole gear ratio thing and don't want to make a bad decision when I buy a posi for it. Just trying to educate myself.

Thank you again for your help.
 
well with a 3:55 at 60 mph your engine will roughly be doing 3,000 rpm depending on the tire size. a 3:36 will lower that rpm some what. So it's up to your priorities, are you more interested in lower engine rpm at highway speeds, a 3:36, or faster off the line, a 3:55.
 
Thank you that is exactly what we are looking for. Gives us an idea of what does what. There are a lot of other combo's out there that I saw. For example, I saw on ebay a 10-bolt posi that says it will fit my firebird with a 3.73 gear ratio. Can you tell me what that would get me, kind of like you did up above. At 60 miles per hour my rpm's would be approx? Is there a way for me to figure that out on my own so I don't have to keep bugging you?
 
The 3.73 will make the car accelerate faster, but will bring the rpm up about 100 at 60.
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Thank you. So from what I am seeing the first number in the ratio (3) remains consistent. The second number is really what changes the ratio and it appears that the lower the second number, the less you will have off the line and the more you will have on the highway. Am I understanding correctly?

3:36 at 60mph = approx. 2900 rpm - slower off the line, but lowest rpm on highway
3:55 at 60mph = approx. 3000 rpm - maybe best of both worlds - good off line and good on highway
3:73 at 60mph = approx. 3100 rpm - quickest off the line but higher rpm on highway

I think I am starting to get the concept.

Thank you!
 
You have it perfectly. Remember you will also have to get the correct gear for your transmission if you want an accurate speedometer
 
Thank you very much for all your help! You have answered our questions. Really appreciate it!
 
Posi

Good Morning,

Thank you again for all the help yesterday but now I have another question.

We found a 10 bolt posi for our firebird at a great price. The gears aren't what we want, which is not big deal, we'll replace them.

Our friend is selling the 10-bolt posi because he wants a 12-bolt posi saying that they are better. Can you enlighten me please on the difference between the 9, 10 and 12 bolt Posi's? Is there any real difference. We will be using our car as an everyday driver, not racing.

Thank you!
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I'm only aware of the 10 and 12 bolt. The 12 bolt is superior to the 10, stronger. You do realize when going from 10 to 12 you are replacing the whole axle assemby not just the differential?
 
I didn't even know that you could make a 10 bolt into a 12 bolt. The gears that are in the 10-bolt are 4:11 and based on our previous posts we don't want that and would change the 10-bolt 4:11 to 3:36 or 3:55. We are under the impression that this could be done easily but could be wrong. New ring and pinion?

My husband is worried that the 10 bolt isn't strong enough and doesn't want to buy if it isn't going to hold up. Says he only wants to do this once! So we were curious if the 10-bolt is a bad rear end or ok if you aren't racing, etc.
 
On street tires the 10 bolt should hold up hold up. If you increase traction, not so much. If you are doing more than street driving, look into a 12 bolt or a 9 inch axle. As for the ratios the numbers stated are all to 1 example 3.73 to 1, 3.55 to 1 etc. Basically the driveshaft turns 3.37 times to 1 revolution of the wheels. hope this helps.
 
Yes, this helps. I'll pass on to the husband and let you know if he has any further questions. Thanks again!
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If you have a 10 bolt with 4:11 gears you can not slap in any 3 series ring and pinion without an adaptor. The 3 series ring and pinion use a different carrier assembly than a 4 series. This is the first time in this exchange that you mentioned the 4:11
 
So are you saying that I can not put 3:55 gears into a rear-end that has 4:11 gears? Or are you saying that I can but I would also have to buy an adapter? Is there any reason that if I can change them with an adapter that I wouldn't want to. We can buy the rear-end with the 4:11 gears very cheap from our friend but if we can't change to 3:55 gears it isn't worth buying it. Thank you.
 
I'm saying you can not install a 3:55 ring and pinion onto a 4:11 carrier assembly without an adaptor. So before buying the new rear end make sure you can find an adaptor to use the 3:55 ring and pinion.
 
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